A split-jaw chuck such as described in my patent 4,200,300 or in my application Ser. No. 040,126 filed July 24, 1980 (now abandonded) has a chuck body that is rotatable about a chuck axis formed with a plurality of angularly spaced and radially extending front guides and a plurality of angularly spaced and radially extending rear guides. Respective front jaw parts are radially displaceable in the front guides and project forwardly and axially from the chuck body and respective rear jaw parts are radially displaceable in the rear guides. Means is provided including at least one operating element that is engageable with the rear jaw parts for radially displacing same jointly in the chuck body in the respective rear guides. In addition respective coupling members are engageable between the rear jaw parts and the respective front jaw parts and are each displaceable into a coupling position engaging both the respective parts for coupling same together for joint radial displacement and a decoupling position for relative radial displacement of the respective parts.
Such an arrangement has the considerable advantage that even though the rear jaw parts are only moved through a relatively short radial stroke by the operating member, which is normally itself the piston rod of a heavy-duty hydraulic ram, the front jaw parts can easily be set at virtually any radial position on the chuck body. Thus the coupling members can be displaced into their decoupling position and the front jaw parts can be displaced to spacing slightly larger than that required to hold a predetermined workpiece. The coupling members are then displaced back into their coupling positions so that the operating member of the chuck, which is normally constituted as a so-called power chuck, can move them sufficiently to clamp the workpiece in place.
As a rule the chuck body of such an arrangement is formed basically of a short cylindrical piece of steel whose one face is machined with radially extending grooves constituting the front guides and whose back face is similarly machined with radially extending grooves constituting the back guides. These grooves all open radially outwardly to facilitate machining of the chuck body and to allow the respective jaw parts to be mounted in place. To this end the jaw parts are typically formed of rectangular section, seen parallel to their respective directions of radial displacement. Once assembled together a relatively thin plate is bolted over the back face of the chuck body to hold the rear jaw parts in place.
Such a system has the disadvantage that the chuck body is occasionally stressed so very much that the relatively thin webs separating the grooves constituting the front guides from the grooves constituting the rear guides fail. In this case the chuck body comes apart, potentially causing serious injury and at the very least destroying a very valuable machine tool and the workpiece being held thereby. Such chucks are subjected to very large forces, as the axial displacement of the operating member is converted into radial displacement and stressing of the jaws and associated structure.